Wireless communication systems may use either Single Input Single Output (SISO) configurations or Multiple Input Multiple Output (MIMO) configurations. SISO systems include a single antenna at the transmitter and a single antenna at the receiver. By contrast, MIMO systems include multiple antennae at the transmitter, and multiple antennae at the receiver. The additional antennae may have features for signal transmission and reception.
For example, the MIMO system may employ Spatial Multiplexing (SM), which enables the MIMO system to transmit different signals on different antennae. SM enables the MIMO system to generally provide greater throughput, because more signals are transmitted at a particular time and/or frequency over the multiple antennae. A MIMO system may transmit additional SM signals at a subsequent time and/or at a different frequency unit. However, when using SM, multiple signals may interfere with each other at the receiver, especially in a highly correlated channel.
As another example, the MIMO system may also employ Space Time Block Coding (STBC), which enables the MIMO system to transmit a space-time coded signal over multiple antennae during a time interval. In other words, the space-time coded signal includes multiple redundant signals that are each transmitted over a different antenna. In this way, if one of the signals is corrupted before it reaches the receiver, a duplicate signal that is transmitted from a different antenna is available to correct or replace the corrupted signal. This is called “diversity.” Moreover, unlike SM, even if the signals are transmitted simultaneously (i.e., at the same time and/or frequency unit), there is no interference at the receiver. This is because the space-time coded signal is orthogonal. In other words, the redundant signals which are part of the space-time coded signal are orthogonal to each other. Therefore, when the redundant signals are received at the receiver, there is no interference.
As yet another example, the MIMO system may also employ Space Frequency Block Coding (SFBC), which enables the MIMO system to transmit the same signal over multiple antennae at a given time, just as STBC. SFBC is similar to STBC in that it is transmitted orthogonally. However, SFBC differs from STBC, in that it sends different signals over different frequency sub-carriers, instead of at later points in time. Accordingly, SFBC enables MIMO to send redundant copies of the signal, along with different signals, all at the same time. As demonstrated by these examples, the use of MIMO can be beneficial.
At the receiver, the MIMO signal is decoded. There are different decoders that may be used to decode the MIMO signal. If the signals are encoded orthogonally, i.e., as STBC or SFBC, there will be no interference for each signal, and a maximum ratio combining (MRC) detector can be employed at the receiver. However, if the signals are not encoded orthogonally, i.e., as SM, then the signals may interfere with each other, and a minimum mean square error (MMSE) or maximum likelihood (ML) detector can be employed at the receiver. Of the three detectors, MRC is simpler than MMSE, and ML requires the largest computational effort.
An optimal decoder to use in any given situation depends on the format of the transmit signal. The optimal decoder is one that produces a decoded signal that reaches the ML, and known as the best solution. For SM formatted transmit signals, the optimal detector may be the ML detector. For STBC/SFBC formatted transmit signals, the optimal detector may be the MRC detector, as the solution to the MRC is already a ML solution. The reason that STBC/SFBC may be compatible with the simpler MRC detector, is because the orthogonal encoding of the symbols in STBC/SFBC cancels the interference among the different signals. However, this is only true given the assumption that a subsequent channel (whether in time or frequency) is slowly or nearly time invariant. In other words, the assumption is that the subsequent channel does not change (or changes very little) according to time or frequency.
To increase reliability, both SISO and MIMO systems may employ a Hybrid Automatic Retransmission Request (HARQ). With HARQ, the receiver performs a cyclic redundancy check (CRC) on the received signal. If the result of the CRC is positive, then the receiver sends an acknowledgement (ACE) to the transmitter. However, if the result of the CRC is negative, the receiver sends a negative acknowledgement (NACK) to the transmitter. After the transmitter receives the NACK, it retransmits at least a portion of the original signal to the receiver, so that the receiver can correct the error in the previously received original signal.
When signals are retransmitted using HARQ, the receiver must combine the retransmitted signal with the original signal in order to correct the error. There are two primary schemes by which to combine these signals. First, the receiver may use bit level combining, whereby the receiver combines the signals at the bit level. Second, the receiver may use symbol level combining, whereby the receiver combines the signals at the symbol level. The symbol is a constellation point mapping of a collection of bits. As used here, a symbol is a representation of a unit of data. Whether using bit level combining or symbol level combining, an initially transmitted signal may be encoded using SM. Moreover, conventional MIMO HARQ systems retransmit the same SM signal when an error is found in the initially transmitted signal. The retransmitted signal may be combined with the initially transmitted signal at the receiver at the bit level. Alternatively, the retransmitted signal may be combined with the initially transmitted signal at the receiver at the symbol level, for example with a joint MMSE detector, and the MMSE detector as discussed above.
In SISO systems, bit level combining and symbol level combining do not differ significantly with respect to performance. However, in MIMO systems, when using the conventional retransmitted SM pattern, using symbol level combining improves the combination performance as compared to bit level combining. This is because a better conditioned equivalent channel may be obtained before detection when using symbol level combining. A channel may be well-conditioned when columns of the channel are substantially orthogonal to each other. In this way, the interference between SM signals can be fully eliminated.
However, symbol level combining has some drawbacks and restrictions. First, symbol level combining consumes more buffer space in the receiver as compared to other methods of combining in the receiver. Reducing buffer consumption may be desirable in MIMO systems, especially when implementing system on a chip (SoC) design. A second drawback is that the retransmitted bits should be aligned in symbol mapping with respect to that of the original transmission. In other words the constellation signal should be aligned to permit the same constellation points. Third, symbol level combining may consume more computation power than other methods of combining. Regardless of whether symbol level combining or bit level combining is used at the receiving side, both the initial packet and the retransmitted packet are customarily sent using the same MIMO pattern format.